I'm new in this forum and I just bought S3D, very nice product.
My first question is : what is the way to program in G-CODE a procedure to do a printing pause at a specific layer, move the hothead, inset manually a nut in a housing, and clic on continue job to finish printing ?

Use the previewer to figure out what layer you want to pause at. Let's say it's layer 153. Then add the following command into your post processing script.

{REPLACE "; layer 153\n" "M0\n; layer 153\n"}

When the print gets to that layer, the print will automatically pause just as if you had clicked the Pause button in the machine control panel. You can then insert your nut, and click Resume to begin printing again.

I have tried, that works the task was stopped and Simplifi3d invite you to press resume to continue the job.
So I lift Z for 10mm, then changed filament, then made extrusion for 100mm to clean the new color.
After that I pressed resume, printer restart the job but extrusion was not giving filament anymore.
I have tested without use extrusion function from joystick panel that works, so use extrusion seems to break something, maybe the last settings about speed for extrusion.
Ah something else, this is not working:
{REPLACE "; layer 153\n" "M0\n; layer 153\n"}
Need to use this:
{REPLACE "; layer 153," "M0\n; layer 153,"}
Thanks for advice

Here's another way to put a metal thread in your 3D print. It would require you to redesign your part, but the installation is extremely easy.

Run some tests to determine what size hole to print. It's best if it is tapered, and strongest if the insert is inserted with the big end away from the screw head.

After printing, position the insert over the hole and press it in with a hot soldering iron--one over 40 watts would be best. In a few seconds, the insert will melt into the plastic. Have a flat piece of wood or metal to press the insert flush with the surface.

Just an additional Tip!
After inserting the nut, then printer will try to print on top of it (closing the gap)... since some nuts are plated, there might not be enough adhesion. these are bridges that S3D is trying to build on top od the nut, but a bit of adhesion definedly helps, specially if you are pritings ABS... so you can just have the nuts heating in the top of the heatbed, on a corner or so (where it's not in the way of the extruder) and you can actually apply a coat of hairspray (for ABS) or PVA (for PLA) and when you need to insert just do it with the nut hot and preped as if it was the metal of the heatbed... this will help a lot the printer when resuming the print.